Kuesu Jinguuji (Omamori Himari)
She's a trained demon hunter, and already has a birthmark shaped like a crescent moon on her forehead.
On the other hand, she has a major chip on her shoulder about her family pride, and believes that the proper way to prove her family's worth to all the other demon hunter clans is the genocidal elimination of all non-human sentients.
Of course, if she's the Moon Princess, then she is fate to marry the Earth Prince. That means she can't marry Yuuto. The cat, the snake, the teacup, and the token normal girl will all be delighted that the contender for who is getting into Yuuto's pants that they all like the least is out of the contest.
If you want to make it even more complicated, make Himari Noihara take the role of Luna.